1. Field of the Invention
The present invention relates generally to a fixing method for fixing a seed crystal to be used for growing a single crystal of, for instance, silicon carbide to a graphite base, and a method of manufacturing a single crystal using the same.
2. Related Background Art
Recently, a silicon carbide single crystal substrate has been developed as a semiconductor substrate for high withstand voltage and high power semiconductor devices such as high withstand voltage and power transistors, high withstand voltage diodes, etc. A sublimation-recrystallization method (the improved Reilly method) mainly is employed as the method of manufacturing the silicon carbide single crystal substrate. FIG. 5 is a schematic view of an apparatus that is used in the sublimation-recrystallization method. A graphite crucible including a container 14 and a lid 15 provided with a base contains SiC powder 16 as raw material powder in its lower half while a seed crystal 17 is placed on the lower surface of the lid 15 facing the SiC powder. The inside of the crucible is maintained with a higher temperature on the SiC powder 16 side and a lower temperature on the seed crystal 17 side. Sublimated gas of the SiC powder 16 recrystallizes on the seed crystal 17 having a lower temperature and thereby a single crystal 18 grows.
In the above-mentioned method, the seed crystal 17 generally is attached to the base provided for the lid 15 using an adhesive. In that case, however, as shown in FIG. 6A, a heat treatment process for drying and curing the adhesive tends to cause air bubbles to be generated in an adhesive layer 19 and thereby voids 20 may remain in the adhesive layer 19. When a single crystal is grown in the presence of the voids 20, heat is conducted to the lid 15 from regions of the seed crystal 17 that closely adhere to the adhesive layer 19 and are not in contact with the voids 20. This results in no temperature gradient produced between the seed crystal 17 and the lid 15. The seed crystal 17, however, does not allow heat to escape to the lid 15 in its regions that are in contact with the voids 20, which results in a temperature gradient produced locally between the seed crystal 17 and the lid 15.
As a result, as shown in FIG. 6B, the seed crystal 17 sublimates from the rear face of the seed crystal 17 to the lid 15 having a lower temperature through the voids 20. Such rear face sublimation that is caused from the rear face of the seed crystal 17 occurs in a plurality of locations in the plane at which the seed crystal 17 and the lid 15 are attached to each other, and it continues during single crystal growth. Accordingly, as shown in FIG. 6C, large defects (macroscopic defects) 21 that propagate in the growth direction from the interface between the seed crystal 17 and the base are caused. The presence of these macroscopic defects 21 not only causes difficulty in cutting out a large number of usable SiC wafers even when a long SiC single crystal is obtained, but also may cause hollow penetration defects called “micro-pipes”, with the macroscopic defects 21 being used as originating points. Hence, there is a problem that it is difficult to obtain a high-quality SiC single crystal with a large area.
In order to solve this problem, for instance, a method of allowing a seed crystal to have a uniform in-plane temperature distribution has been proposed that is carried out by joining a seed crystal and a base of a lid to each other, with a carbonized layer being interposed therebetween (see JP9(1997)-110584A; Patent Document 1). This configuration is the same as that shown in FIGS. 6A to 6C except that the adhesive layer 19 is replaced by the carbonized layer 22. Hence, the illustration thereof is omitted. Furthermore, a method of inhibiting the development of macroscopic defects has been proposed that is carried out by coating the surfaces of the seed crystal, other than the surface on which a single crystal grows, with a protective layer formed of a material (for instance, tantalum) that is stable under single crystal growth conditions, and then joining the seed crystal to a base of a lid with an adhesive (see JP9(1997)-268096A; Patent Document 2).
In the method described in Patent Document 1, however, a liquid adhesive (for instance, a resist) containing a polymeric material is provided between the seed crystal 17 and the base of the lid 15 and then is heat-treated at high temperature to form the carbonized layer 22 therebetween to fix the seed crystal 17. Accordingly, voids 20 tend to be produced due to air bubbles generated by the heat treatment process. As a result, the development of the macroscopic defects 21 cannot be inhibited completely as in the case shown in FIG. 6C.
Similarly, in the method described in Patent Document 2, as shown in FIGS. 7A to 7C, unevenness of the adhesive layer 19 causes local temperature distribution. As a result, cracks 24 are caused in the protective layer 23 and thereby the rear face sublimation occurs through the cracks 24 between the seed crystal 17 and the surface of the base of the lid 15 to which the adhesive layer 19 adheres. This produces macroscopic defects 21, which has been a problem.